A plastic sandwiched board comprising a core of a foamed or unfoamed plastic material and surface layers of a metal or a rigid plastic material such as a glass fiber-reinforced plastic material, laminated on the core, is widely used as a light-weight composite material having excellent rigidity and impact resistance for the production of a car body, a construction panel, a top structure of a ship and the like. These plastic sandwiched boards have heretofore been prepared according to a method comprising heat-pressing surface layers to a pre-formed plastic core by using an adhesive. Accordingly, the productivity was low.
As means for solving this defect, there has been proposed a method in which a reaction liquid containing a monomer having a norbornene unit and a metathesis catalyst is cast between spaced surface layers and polymerized according to the reaction injection molding technique to obtain a plastic sandwiched board (see Japanese Unexamined Patent Publication No. 62-122718). According to this method, a plastic sandwiched board can be manufactured more easily than by the conventional method, and light-weight composite material having excellent mechanical properties can be obtained. However, in this method, since metal sheet or a rigid plastic sheet is used, the adhesion to the polymer of the core is insufficient, and it is difficult to color the surface layers the desirable color or freely control the thickness of the surface layers, or to laminate the surface layers in the state registered with the shape of the core when the shape of the core is complicated. The rigid plastic sheet can be in the form of a sheet of plastic material such as epoxy resin, phenolic resin or an unsaturated polyester reinforced with glass or carbon fibers.
A molded article obtained by bulk-polymerizing a norbornene monomer such as dicyclopentadiene (DCP) by the reaction injection molding (RIM) method is generally colored yellow or brown, and even if a pigment is incorporated, the color tends to change with the lapse of time. Even if it is intended to improve or modify the surface state of the molded body by laminating a colored or printed plastic sheet, a composite material having sufficient utility is difficult to obtain because most of these colored or printed plastic sheets have poor adhesion to the bulk-polymerized product of a norbornene monomer.
Furthermore, in molding a norbornene monomer according to the RIM method or the like, the unreacted monomer is often left in the molded article, and hence, a peculiar smell is generated and use of the molded article is thus restricted.